Radar devices radiate a radio-frequency radar transmission signal to the space from a measuring site, receive a reflection wave signal reflected from a target, and measure at least one of a distance between the measuring site and the target and a direction of the target. In recent years, radar devices which can estimate a distance to or a signal incoming direction from a target that may be an automobile or a pedestrian by performing a high-resolution measurement using a short-wavelength radar transmission signal such as a signal of microwaves or millimeter waves have been being developed.
Radar devices receive a signal that is a mixture of reflection waves coming from a nearby target and reflection waves coming from a distant target. Range sidelobes occur due to a reflection wave signal coming from a nearby target. If range sidelobes and a main lobe of a reflection wave signal coming from a distant target exist in mixture, the accuracy of detection of the distant target may be lowered.
Therefore, radar devices which are required to perform high-resolution measurement on plural targets are required to transmit a pulse wave or a pulse-modulated wave using a transmission code that has an autocorrelation characteristic with low range sidelobe levels (hereinafter referred to as a low range sidelobe characteristic).
When an automobile and a pedestrian are located at the same distance from a measuring site, a radar device receives a signal that is a mixture of signals of reflection waves coming from the automobile and the pedestrian which have different radar cross sections (RCSs). The radar cross section of a pedestrian is smaller than that of an automobile.
Radar devices are required to properly receive reflection wave signals coming from an automobile and a pedestrian even if they are located at the same distance from a measuring site. Since the signal level of a reflection wave signal varies depending on the distance or type of a target, radar devices are required to have a reception dynamic range wide enough to enable reception of reflection wave signals of various signal levels.
One example conventional radar device is known which transmits pulse waves or pulse-modulated waves through mechanical antenna scanning or electronic scanning with a narrow-directivity beam and receives a reflection wave signal reflected from a target.
Another example conventional radar device is known which receives a reflection wave signal reflected from a target by plural antennas and measures reception phase differences between received reflection wave signals, thereby estimating a signal incoming angle using resolving power that is higher than a value corresponding to beam directivity of each antenna.
In the former radar device, a long antenna scanning time is necessary for detection of a target because it transmits and receives radio waves using the single antenna. Therefore, it is difficult to detect a target so as to follow its movement because detection of a fast-moving target necessitates a high-resolution measurement and hence many scans.
The latter radar device can attain higher detection accuracy than the radar device using a single antenna because it can estimate a signal incoming direction by performing signal processing with decimated scanning intervals. Furthermore, the latter radar device can estimate a signal incoming angle following a movement of even a fast-moving target.
Patent document 1, for example, is known as disclosing a conventional technique in which reflection wave signals that exhibit high correlation are separated from reflection waves received by plural antennas and used for incoming direction angle measurement calculation. In the super-resolution antenna of Patent document 1, reception signals received by plural antennas are subjected to Fourier transform and signal components of respective filter banks corresponding to respective Doppler frequencies generated by the Fourier transform are weighted. In this super-resolution antenna, a desired filter band component is extracted from the weighted signal components and a signal incoming direction from a target is estimated on the basis of the signal component corresponding to the extracted element antenna.
With the above technique, if plural targets exist within a distance resolution, the targets are high in correlation, and Doppler frequencies of the respective targets are sufficiently different from each other, reception signals from the respective targets can be separated from each other and signal incoming directions from the respective targets can be estimated.